What is this yellow line with a dot at the end?
You've simply activated a tool from the toolbox on the left. You can see wich tool you have selected by the blue background on it.
If you don't see that toolbox, simply hit T while your cursor is hovering over your 3D viewport, or drag the little arrow on the top-left part of the viewport.

The default tool is Rectangle Selection (the top-most one), click it.